A rechargeable battery is damaged or ruined by charging past full charge at other than a trickle charge rate. The problem has become more acute since rapid recharge has come into use. For example, in place of 10 hour rate charging for sealed batteries (e.g., nickel-cadmium), new high rate recharge techniques of one hour or less are under investigation and may come into wide use by government and industry. The exothermic reaction in a battery being charged beyond full charge, particularly at high rate recharge can result in an explosion. An inefficient method of avoiding overcharging is for the technician to estimate the degree of battery discharge and then to decide on a safe charging period so as to terminate the charging with a conservative margin of safety, that is, substantially before the battery is fully charged.